This is a single-arm, open-label, multicenter study of Iodine-131 Anti-B1 Antibody for the treatment of non-Hodgkin's lymphoma (NHL) patients who were previously treated with rituximab antibody. Forty patients will receive study drug on this study. Patients must have been treated with at least four doses of rituximab and have progressed during of following rituximab therapy. Patients will undergo two phases of study. In the first phase, termed the "dosimetric dose", patients will receive an infusion of unlabeled Anti-B1 Antibody (450 mg) over 70 minutes (including a 10-minute flush) immediately followed by a 30-minute infusion (including a 10-minute flush) of Anti-B1 Antibody (35 mg) which has been radiolabeled with 5 mCi of Iodine-131. Whole body gamma camera scans will be obtained on Day 0, Day 2, 3 or 4, and Day 6 or 7 following the dosimetric dose. Using the dosimetric data from the three imaging timepoints, a patient-specific dose of Iodine-131 to deliver the desired total body dose of radiotherapy will be calculated. In the second phase, termed the "therapeutic dose", patients will receive a 70-minute infusion (including a 10-minute flush) of unlabeled Anti-B1 Antibody (450 mg) immediately followed by a 30-minute infusion (including a 10-minute flush) of 35 mg Anti-B1 Antibody labeled with the patient-specific dose of Iodine-131 to deliver a whole body does of 75 cGy to patients with no hematologic risk factors. Patients who have baseline platelet counts of 100,001 - 149,999 cells/mm3 will receive 65 cGy and patients who are obese will be dosed based upon 137% of their lean body mass. Patients will be treated ;with saturated solution potassium iodide (SSKI), Lugol's solution, or potassium iodide tablets, starting at least 24 hours prior to the first infusion of the Iodine-131 Anti-B1 Antibody (i.e., the dosimetric dose) and continuing for 14 days following the last infusion of Iodine-131 Anti-B1 Antibody (i.e., the therapeutic dose). The endpoints of the study are to determine the response rate, complete response rate, duration of response, time to progression, time to treatment failure, safety, and survival of Iodine-131 Anti-B1 Antibody therapy in NHL patients who have previously been treated with rituximab.